1. Field of the Invention
The present invention relates to a method and an apparatus for inspecting surface defects on the magnetic disk file memories and in particular to a method and an apparatus for inspecting surface defects of magnetic disks, whereby defects present on the surface of the magnetic disk having minute aventurine prominences and dents can be automatically inspected.
2. Description of the Prior Art
A magnetic disk is obtained by applying magnetic oxidized steel grains onto a polished and finished aluminium surface. In the application process, a mixture liquid composed of a solvent and oxidized steel grains is dropped onto an aluminium plate rotating at high speed and thereafter the solvent is evaporated. On the film thus formed in the application process, craterlike defects and stripe defects are caused.
This phenomenon will now be described in detail by referring to the following drawings.
FIG. 1 is a plan view of a magnetic disk. FIG. 2 is an enlarged oblique view of a portion II cut away from the disk illustrated in FIG. 1. FIG. 3 is an enlarged oblique view of a portion III cut away from the disk illustrated in FIG. 1, FIG. 4 is an enlarged sectional view of a typical defect appearing on the magnetic disk.
In those figures, reference numeral 1 denotes a magnetic disk having a film layer 5 formed by applying magnetic oxidized steel grains onto an aluminium plane 4 thereof. Reference numerals 2 and 3 respectively denote a craterlike defect and a stripe defect caused on the film layer 5. If a portion of the aluminium plane 4 where it contacts with the mixture liquid is not pure, the craterlike defect 2 appears on the surface of the film layer 5 after the solvent has been evaporated. If the grain density is not uniform somewhere, the stripe defect 3 is caused on a radially outer end of the aluminium plane 4 by the centrifugal force due to rotation caused by spinner.
Since the craterlike defect 2 and the stripe defect 3 might cause a read/write error of the magnetic disk file memory 1, it is necessary to effect inspection to see whether such a defect exists or not after the application process has been completed.
The aluminium plane 4 is 2 mm in thickness and the film layer 5 formed on the plane 4 is 1 .mu.m in thickness. The film layer 5 has minute aventurine dents and prominences which are 0.01 .mu.mRa in surface roughness. The size of the craterlike defect 2 and the width of the stripe defect 3 (hereafter collectively referred to as size d) are typically 0.5 to 2 mm as illustrated in FIG. 4. Since a defect more than 0.2 .mu.m in depth h might cause a read write error, the depth h of a defect more than 0.2 .mu.m must be detected. Accordingly, the depth h of a defect to be detected is extremely small in relation to the size d as represented by the ratio as EQU h/d=1:2500 (.about.10,000).
Such a small defect can be detected by using interference fringes such as Newton's rings, for example, if the surface is optically smooth. However, the Newton's rings cannot be applied to the surface having minute aventurine dents and prominences such as the surface of the magnetic disk file memories. Such a surface has heretofore been visually inspected, resulting in the low reliability of the magnetic disk performance if a human inspector missed the defects.